SMC Networks SMC6624M Management Manual page 337

1. If VLAN_1 (VID=1) is configured as "Untagged" on port 3 on switch "X",
then it must also be configured as "Untagged" on port 7 on switch "Y".
Make sure that the VLAN ID (VID) is the same on both switches.
2. Similarly, if VLAN_2 (VID=2) is configured as "Tagged on the link port on
switch "A", then it must also be configured as "Tagged" on the link port
on switch "B". Make sure that the VLAN ID (VID) is the same on both
switches.
Duplicate MAC Addresses Across VLANs. Duplicate MAC addresses on
different VLANs are not supported and can cause operating problems. There
are no explicit events or statistics to indicate the presence of duplicate MAC
addresses in a VLAN environment. However, one symptom that may occur is
that a duplicate MAC address can appear in the Port Address Table of one
port, and then later appear on another port. (This can also occur in a LAN
where there are redundant paths between nodes and Spanning Tree is turned
off.) For more information, refer to "VLAN Restrictions" on page 9-72.
Troubleshooting
Unusual Network Activity
11-9